1. Field of the Invention
The present invention is generally directed to a measurement embodiment, and particularly, to a measurement embodiment that eliminates temperature conditioned inaccuracies by creating a resulting thermal expansion equal to the machine part thermal expansion.
2. Description of the Prior Art
A variety of differently constructed measuring embodiments exist. A measurement embodiment consisting of a base body, a graduation carrier and a graduation or scale joined thereto is used in a position measuring arrangement for measuring the relative positions of two objects, for example a machine tool and a workpiece. One object is joined with the measurement embodiment which includes a graduation. A scanning unit attached to the other object scans the graduation of the measurement embodiment. Typically, a graduation carrier is used to join a graduation to a machine part. The graduation is scanned to determine precise movements of the machine.
One problem with attaching a graduation carrier to a machine piece is the change in temperature due to the operating conditions. More specifically, the graduation carrier, usually composed of glass, and the machine piece, usually composed of steel, have different thermal expansion coefficients. The material for the carrier is preferably glass since glass can be produced economically and it has excellent optical properties. The rising temperature during the operation of the machine leads to different dimensional changes of the various components due to the different expansion coefficients. Since the machine piece, the graduation carrier and the graduation will usually not have the same thermal expansion coefficients, the temperature changes resulting from operation leads to inaccuracies in measurement.
There have been various suggestions for correcting the problem. German Utility Model No. 7,513,496 describes a compensating device in which inaccuracies of the machine are compensated. A glass scale is cemented into a hollow aluminum section with a highly elastic layer being placed between the scale and the hollow section. The hollow section is fastened to the machine by means of special end pieces. The highly elastic layer serves to mechanically uncouple the glass scale from the hollow aluminum section. Thus, constraining forces cannot be transmitted from the machine to the glass scale. The compensating device acts on the end faces of the glass scale and can compress or stretch the glass scale in accordance with the requirements of the temperature change. Accordingly, the compensating device adjusts the effective length of the graduation.
In addition, fastening a scale in a measuring device in a floating manner has been known at least since the disclosure in German Patent No. 1,176,382. This method is now considered necessary in order to fasten the highly accurate graduation free of constraining forces. A number of other publications show that it has been considered necessary in the art to uncouple the graduation from the machine, so that no thermally related constraining forces influence the graduation. This view is expressed, for example, in German Offenlegungsschrift No. 2,016,253 and U.S. Pat. Nos. 3,816,002 and 3,629,945.
U.S. Pat. Nos. 4,793,049 and 4,835,830 disclose a measurement embodiment which consists of a base body and a graduation carrier with a graduation applied thereto. The graduation carrier member has a smaller temperature expansion coefficient than the base member. The carrier is placed on the base in a floating manner at normal temperature. Then the carrier and the base are heated. The end faces of the graduation carrier are then fixed by means of fastening elements to the base member in such a way that the graduation carrier member is in a compressed state when cooled to a normal temperature. In order to obtain the correct graduation, however, the graduation marks must be made with increased spacing between marks. The increase in the spacing depends upon the ratio of the temperature coefficients of the base member and the graduation carrier and the magnitude of the desired heating temperature. The production of such a measurement device is expensive.
Accordingly, it is a primary object of the present invention to provide a close linking connection between the graduation carrier and the base body by using a galvanically deposited interlayer.
Another object of the present invention is to provide a measurement embodiment that undergoes the same temperature conditioned length changes as the machine part.
Further objects and advantages will become apparent from the following description and the accompanying drawings.